Really gorgeous: a fully mature flatheaded mayfly, Leucrocuta hebe. Location: Doyles River up at Blackwells Hollow Road. Some more photos and then we'll go into the species ID.
"L. aphrodite and L. hebe may have a pale medial V-shaped area on abdominal terga 7, 8, and most of the medial length of 9, or on tergum 9, three connected or unconnected pale spots (possibly even one medial spot in extreme variants). Sternal and caudal filament markings that have been associated with either of these species vary considerably between and among populations, and other characteristics that may be of diagnostic value are not known with confidence. For example, L. hebe might have an unmarked ventral abdomen or one with pairs of ventral spots in some populations, and also commonly a pair of posterolateral spots on sternum 9, or extensive lateral staining on 9." (Larvae of the Southeastern USA: Mayfly, Stonefly, and Caddisfly Species, p. 110)
What I see on our nymph is this. 1) There is clearly a "V" (or a "U") on tergum 7, 8 is pretty solid in color medially, and on 9, I'd say there are 3 pale unconnected spots.
This is the venter.
The ventral abdomen on our nymph is "unmarked" while on segment 9, there is, I think, lateral staining. I think L. hebe is the right call.
Nymph number two -- a fully mature pronggilled mayfly, Paraleptophlebia guttata.
I wasn't sure of the species ID on this little guy (also 4 mm) -- the pale terga were a little unexpected -- so I decided to key it out. P. guttata for sure. 1) "Gills 2-7 forked near base, usually not more than one-sixth length from base; gill trachea without distinctly pigmented lateral branches." (Larvae of the Southeastern USA, p. 140).
2) "Combined length of distal two maxillary palp segments about 1.5x or more length of proximal segment." (p. 140) Segments 2 and 3 were 1.6 times as long as segment 1.
3) "Posterolateral projections present on abdominal segment 9 only." (p. 140) Yes.
and 4) "Abdominal terga 2-6 laterally without blackish bands (as on P. strigula), but often with diffuse blackish spot in posterolateral corners." (p. 141) Yep, those spots are there.
Beautiful day in Virginia. Stream level was great. But now we're on the lookout for Hurricane Irma!